1. Field of the Invention
The present invention relates to ink-jet recording. More specifically, the present invention relates to an ink set, an ink-jet recording process and a recording apparatus, by which high-quality color images can be formed on plain paper.
2. Related Background Art
Ink-jet recording is a method in which recording is conducted by ejecting droplets of an ink and then applying the droplets to a recording material such as paper. In particular, according to the methods disclosed in Japanese Patent Publication Nos. 61-59911, 61-59912 and 61-59914 by the present applicant, in which an electrothermal converter is used as an ejection-energy supply means to apply thermal energy to an ink so as to generate bubbles, thereby ejecting droplets of the ink, formation of a high-density multi-orifice head can be realized with ease, and high-resolution and high-quality images can be recorded at high speed.
However, the conventional inks used in ink-jet recording generally comprise, as a principal component, water, and include additionally a water-soluble high boiling solvent such as glycol for the purpose of preventing drying and clogging, and the like. When such an ink has been used to conduct recording on plain paper, there have been disadvantages that the ink penetrates into the interior of the recording paper, resulting in failure to achieve sufficient image density, and moreover, unevenness of image density occurs, which appears to be attributed to the uneven distribution of the filler and size on the surface of the recording paper. In particular, when color images are intended to form, a plurality of inks of different colors are overlapped one after another before they are fixed. Therefore, color bleeding and uneven color mixing have occurred at portions of boundaries between images of different colors (hereinafter referred to as "bleeding" simply), resulting in failure to obtain satisfactory images.
As means for solving the above-described problems, Japanese Patent Application Laid-Open No. 55-65269 discloses the use of inks comprising a compound capable of enhancing their penetrability, such as a surfactant, and Japanese Patent Application Laid-Open No. 55-66976 discloses the use of inks comprising, as a principal component, a volatile solvent. However, the former method has involved a disadvantage that although the penetrability of the inks into recording paper is improved, and the bleeding can be prevented to some extent, the image density and the brightness of images are lowered because the inks penetrate deeply in the recording paper together with coloring matter. In addition, since the wettability to the surface of the recording paper is also improved, the inks tend to spread, resulting in reduction of resolution and occurrence of feathering. Therefore, such a method has not been preferred. In particular, this problem is noticeable when black-colored characters are expressed. On the other hand, the latter method is liable to cause clogging due to the evaporation of the solvent in a nozzle of a recording head in addition to the above-described disadvantages. Such a method has therfore not been preferred.
Further, there have been disclosed methods of applying a liquid, which can make the quality of images better, to recording paper prior to the jetting of a recording ink. Japanese Patent Application Laid-Open No. 63-29971 discloses a method in which a liquid containing an organic compound having two or more cationic groups per molecule is applied to recording paper, and recording is then conducted with an ink containing an anionic dye. Japanese Patent Application Laid-Open No. 64-9279 discloses a method in which an acidic liquid containing succinic acid or the like is applied to recording paper, and recording is then conducted with an ink. Japanese Patent Application Laid-Open No. 64-63185 discloses a method of applying a liquid, which insolubilizes dyes, to recording paper prior to recording. All these methods intend to improve the fixability, waterproofness and the like of images recorded owing to the deposition of the dye itself in the ink. Although all these methods have been able to prevent bleeding to some extent, they have been poor in coverability on paper fibers in the recording paper because the dye is deposited unevenly on the recording paper, resulting in an image in which white voids are conspicuous. In addition, the rub-off resistance of the image formed has also been poor. Therefore, the image has come off when rubbed with a finger, and the recording paper has been smeared. Accordingly, these methods have not been preferred.